There has recently been a modular multilevel cascade converter (MMCC) as a next generation transformer-less power converter that is suitable for high-voltage and large-capacity application. The MMCC is characterized in that a “cluster” (also called as an arm or a leg in some cases) configuring the converter is formed by cascade connection of unit cells. The representative unit cells include a chopper cell CC illustrated in FIG. 1A and a bridge cell BC illustrated in FIG. 1B.
The chopper cell CC illustrated in FIG. 1A can be regarded as a part of a bidirectional chopper, and includes two semiconductor switches SW that are connected in series, a direct current (DC) capacitor C that is connected in parallel with the two semiconductor switches SW, and input and output terminals T1 and T2 of a current that is discharged from the DC capacitor C or charged in the DC capacitor C according to switching operation of the semiconductor switches SW. The semiconductor switch SW in this example is configured by an IGBT. FIG. 1C illustrates a cluster CL in which a plurality of the chopper cells CC illustrated in FIG. 1A are cascade-connected.
The bridge cell BC illustrated in FIG. 1B is equivalent to a single-phase full-bridge converter, and includes parallelly connected two pairs of two semiconductor switches SW that are connected in series, a DC capacitor C that is connected in parallel with the two pairs of semiconductor switches SW, a series connection point for each pair of the two semiconductor switches SW, and input and output terminals T1 and T2 of a current that is discharged from the DC capacitor C or charged in the DC capacitor C.
The MMCC can be roughly classified into the star-connected MMCC and the delta-connected MMCC according to the connection method. The following six types of the star-connected MMCC and the delta-connected MMCC have been known hitherto, four types of the star-connected MMCC and the delta-connected MMCC of which are disclosed in Non Patent Literature 1.
1. Single star-connected bridge cell MMCC (SSBC)
2. Double star-connected bridge cell MMCC (DSBC)
3. Double star-connected chopper cell MMCC (DSCC)
4. Triple star-connected bridge cell MMCC (TSBC)
5. Single delta-connected bridge cell MMCC (SDBC)
6. Double delta-connected bridge cell MMCC (DDBC)
The applications of the star-connected MMCCs are described herein. The SSBC is applicable to a static synchronous compensator (STATCOM) and a battery power storage device. The DSBC and the DSCC can connect a DC power supply between neutral points of their star connections, and thus can realize DC to three-phase alternating current (AC) power conversion. When the DSBC is used, it is possible to replace the DC power supply with a single-phase AC power supply and is possible to realize single-phase AC to three-phase AC power conversion. The TSBC can connect a three-phase power supply (or a three-phase load) between neutral points of its star connections, and thus can realize three-phase AC to three-phase AC power conversion. Since the star-connected MMCCs are irrelevant to the present invention, further description therefor is omitted.
Next, the applications of the delta-connected MMCCs are described. As illustrated in FIG. 2A, the SDBC includes delta-connected three clusters CL in each of which a plurality of bridge cells BC are cascade-connected, with the three connection points of the delta connection being connected to the respective phases of the three-phase AC power supplies. In addition, FIG. 3 illustrates the detail of a circuit configuration in each of the clusters of the SDBC illustrated in FIG. 2A. Since the SDBC can control negative-sequence reactive power by controlling a circulating current in the delta connection, it is expected to be applied to a negative-sequence reactive power compensation apparatus for an electric arc furnace.
In a single delta-connected bridge cell MMCC 100 (hereinafter, referred to as a power converter 100) illustrated in FIG. 3, the phase voltages of the respective phases of the system-side power voltage are defined as vSu, vSv, and vSw, and the currents of the respective phases (hereinafter, referred to as “power currents”.) are defined as iu, iv, and iw. In addition, the currents that flow into the clusters CL of the respective phases from the delta connection portions of the power converter 100 (hereinafter, referred to as “converter currents”) are defined as iuv, ivw, and iwu. In addition, the output voltages of the clusters CL of the respective phases from the delta connection portions of the power converter 100, i.e., the line voltages between output terminals TU1-TU2, TV1-TV2, and TW1-TW2 of the power converter 100 are defined as vuv, vvw, and vwu. In addition, the voltages of the DC capacitors in the bridge cells 11u-j, 11v-j, and 11w-j are defined as vCju vCjv, and vCjw (where j=1 to 3).
On the other hand, as illustrated in FIG. 2B, the DDBC includes six clusters CL in each of which a plurality of bridge cells BC are cascade-connected. In the double delta connection, pairs of two clusters CL that are connected in series are delta-connected, and the three connection points of the delta-connected pairs are respectively connected to the U phase, the V phase, and the W phase of the three-phase AC power supplies. The intermediate points of the serially connected clusters CL are respectively extracted as the R phase, the S phase, and the T phase. Thus, the DDBC is capable of realizing three-phase AC to three-phase AC power conversion in the same way as the TSBC.